TZSET(3) Linux Programmer's Manual TZSET(3)
NAME
tzset - initialize time conversion information
SYNOPSIS#include<time.h>voidtzset(void);externchar*tzname[2]DESCRIPTION
The tzset() function initializes the tzname variable from
the TZ environment variable. This function is automati-
cally called by the other time conversion functions that
depend on the time zone.
If the TZ variable does not appear in the environment, the
tzname variable is initialized with the best approximation
of local wall clock time, as specified by the
tzfile(5)-format file /etc/localtime.
If the TZ variable does appear in the environment but its
value is NULL or its value cannot be interpreted using any
of the formats specified below, Coordinated Universal Time
(UTC) is used.
The value of TZ can be one of three formats. The first
format is used when there is no daylight saving time in
the local time zone:
stdoffset
The std string specifies the name of the time zone and
must be three or more alphabetic characters. The offset
string immediately follows std and specifies the time
value to be added to the local time to get Coordinated
Universal Time (UTC). The offset is positive if the local
time zone is west of the Prime Meridian and negative if it
is east. The hour must be between 0 and 24, and the min-
utes and seconds 0 and 59.
The second format is used when there is daylight saving
time:
stdoffsetdst[offset],start[/time],end[/time]
There are no spaces in the specification. The initial std
and offset specify the standard time zone, as described
above. The dst string and offset specify the name and
offset for the corresponding daylight savings time zone.
If the offset is omitted, it defaults to one hour ahead
of standard time.
The start field specifies when daylight savings time goes
into effect and the end field specifies when the change is
made back to standard time. These fields may have the
following formats:
Jn This specifies the Julian day with n between 1 and
365. February 29 is never counted even in leap
years.
n This specifies the Julian day with n between 1 and
365. February 29 is counted in leap years.
Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <=
w <= 5) of month m (1 <= m <= 12). Week 1 is the
first week in which day d occurs and week 5 is the
last week in which day d occurs. Day 0 is a Sun-
day.
The time fields specify when, in the local time currently
in effect, the change to the other time occurs. If omit-
ted, the default is 02:00:00.
The third format specifies that the time zone information
should be read from a file:
:[filespec]
If the file specification filespec is omitted, the time
zone information is read from /usr/lib/zoneinfo/localtime
which is in tzfile(5) format. If filespec is given, it
specifies another tzfile(5)-format file to read the time
zone information from. If filespec does not begin with a
`/', the file specification is relative to the system time
conversion information directory /usr/lib/zoneinfo.
FILES
/usr/lib/zoneinfo system time zone directory
/usr/lib/zoneinfo/localtime local time zone file
/usr/lib/zoneinfo/posixrules rules for POSIX-style TZ's
CONFORMINGTO
SVID 3, POSIX, BSD 4.3
SEEALSOdate(1), gettimeofday(2), time(2), ctime(3), getenv(3),
tzfile(5)
BSD July 2, 1993 1